Hey if you really wanted 2 be sure if it was heat you should run ATI Tool on it and heat up the GPU a bit to see if it resets. Also maybe its CPU heat or a combination of cpu/gpu heat. I would run orthos while running ATI Tool to see if it resets itself.

To me a driver issue wouldn't be the no.1 possibility but i wouldn't rule it out either. Drivers can often cause IRQ errors. Make sure his windows isn't set to automatically boot on a blue screen so that you're missing out on the stop error.